Increased in-field critical current density in neutron-irradiated MgB ₂ films

We study the critical current density Jc of a series of MgB 2 epitaxial thin films with disorder introduced in a controlled way by neutron irradiation. We find that the in-field Jc is significantly increased for an intermediate neutron fluence of ∼7.6 × 10 17cm-2, reaching ∼1.8 × 105Acm -2 at μ0H = 4T, which is larger by more than one order of magnitude than the critical current density of the pristine sample. Moreover, in the same irradiated sample, Jc(μ0H = 9T) is still as large as 1.1 × 104Acm-2. By transmission electron microscopy, we find evidence of the presence of amorphous regions created by irradiation whose average size matches the coherence length ξab. Analysis of the shape of the critical current density and pinning force curves indicates that such regions with a suppressed superconducting order parameter behave as effective point-like pinning centers for magnetic flux lines.